Replace free-form dimension calculation with closest-match from predefined
standard sizes to avoid invalid resolution errors from the API.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
- Support --ref with OpenAI GPT Image models (gpt-image-1.5)
- Auto-select Google or OpenAI when --ref provided
- Change ref-related warnings to explicit errors with fix hints
- Add reference image validation before generation
- Improve retry logic to skip non-retryable errors